The field of radiation hardening in integrated circuits deals with the problem of electrons and holes generated by the passage of ionizing radiation through the circuit. Electron-hole pairs generated in the bulk silicon itself do not present a severe problem, as the electrons and holes recombine rapidly. Electron-hole pairs formed in silicon-dioxide Si-O.sub.2 (referred to be the conventional term of "oxide"), however, are more difficult to deal with because the electrons are far more mobile than the holes and may become separated from the holes, making recombination more difficult.
The conventional process for making FET integrated circuits uses a field oxide between the active regions. Conventionally, the area that will be the field region is implanted either before or after the growth of the field oxide with an ion dose that is calculated to suppress the formation of parasitic transistors. The field oxide is conventionally grown by a wet thermal process, using a locos process or variations of it. With the field implant process, there is a dopant gradient extending down into the substrate, with a high concentration at the surface changing to the background bulk concentration at a depth of a micron or so. The nature of the bulk silicon underlying the circuit will depend on the nature of the process used for the circuit NMOS, PMOS or CMOS process.